This invention relates generally to photovoltaic solar cells, and more particularly the invention relates to a solar cell structure which is efficient in operation and economical to manufacture.
The use of photovoltaic cells for the direct conversion of solar radiation into electrical energy is well known, see Swanson, U.S. Pat. No. 4,234,352 for example. Briefly, the photovoltaic cell comprises a substrate of semiconductive material having a p-n junction defined therein. In the planar silicon cell the p-n junction is formed near a surface of the substrate which receives impinging radiation. Radiated photons create mobile carriers (holes and electrons) and the substrate which can be directed to an electrical circuit outside of the cell. Only photons having at least a minimum energy level (e.g., 1.1 electron volt for silicon) can generate an electron-hole pair in the semiconductor pair. Photons having less energy are either not absorbed or are absorbed as heat, and the excess energy of photons having more than 1.1 electron volt energy (e.g. photons have a wavelength of 1.1 μm and less) create heat. These and other losses limit the efficiency of silicon photovoltaic cells in directly converting solar energy to electricity to less than 30%.
Solar cells with interdigitated contacts of opposite polarity on the back surface of the cell are known and have numerous advantages over conventional solar cells with front side metal grids and blanket or grid metallized backside contacts, including improved photo-generation due to elimination of front grid shading, much reduced grid series resistance, and improved “blue” photo-response since heavy front surface doping is not required to minimize front contact resistance because there are no front contacts. In addition to the performance advantages, the back-contact cell structure allows simplified module assembly due to coplanar contacts. See Swanson U.S. Pat. No. 4,927,770 for example.
While interdigitated back-contact (IBC) solar cells have been fabricated, cost considerations have limited commercialization of the IBC solar cell. Heretofore, conventional microelectronics (integrated circuit) processing has been employed in fabricating IBC solar cells, including the use of backside diffusions, contacts, and metal lines fabricated by conventional microelectronics photolithography, thin film metallization, and etching processes. This fabrication process is capable of producing high efficiency solar cells, but the process is not cost effective for application in conventional low-cost, flat-plate solar panels. The key problem with practical realization of an IBC solar cell by this process is the high cost of fabrication, including etching, doping and mask alignment, and the use of thick metal conductor deposition by vacuum evaporation or sputtering. Further, the processing must be carried out in a clean room environment. Thus IBC solar cells fabricated using these methods have been restricted to application in high concentration solar cells or in very high value one-sun applications.
Polymers are known which can provide electrons and holes when stimulated by radiation. In a pair of coupled donor-acceptor conjugated polymer chains, it is possible for an exciton photo-excited on either polymer to decay and to a hole in the donor polymer surveillance band and an electron in the conduction band of the acceptor polymer. See Wu et al. “Theory of Photo-induced Charge Transfer in Weakly Coupled Donor-Acceptor Conjugated Polymers” Chem. Phys. 227,11 (1998). Bulk photovoltaic have been fabricated using blends of conjugated polymers. See for example Shaheen et al. “Polymer Based Nanostructure Donor-Acceptor Heterojunction Photovoltaic Devices,” NCPC and Solar Program Review Meeting 2003.
The present invention utilizes donor and acceptor polymers in a semiconductor base solar cell to provide a cost effective back contact solar cell.